Transcript Transport and dispersion patterns of aerosols over the East

TRANSPORT AND DISPERSION
PATTERNS OF AEROSOLS OVER THE
EAST AFRICAN REGION
MBITHI D, M., MUTHAMA, J. N., OUMA, G. O.,
O P I J A H , F. O
T H E W O R L D W E AT H E R O P E N S C I E N C E
CONFERENCE 2014
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PRESENTATION OUTLINE
Introduction
Area of Study
Climate Controls
Data
Methodologies
Analysis
Conclusion
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INTRODUCTION
Aerosols are tiny particles suspended in the
atmosphere and can be in the solid phase or
in the liquid phase
They have the capacity to cool or warm the
surface, favor or inhibit the formation of clouds
depending upon their size, type and location,
and, if inhaled, some aerosols are harmful to
people’s health (Platnick and Hubanks, 2009)
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Increasing aerosol concentrations worldwide
have appreciable effects on air quality,
visibility and local, regional and global
climates (Ramanathan et al., 2007)
Simulation of the transport and dispersion
patterns of AOD during different seasons of
the year helps to identify the possible sources
and sinks of the aerosols
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Area of Study
The topographical
features are complex
and include Great Rift
Valley, the East
African highlands,
mountains and
plateaus (Adapted
from Obara, A. D.,
2003)
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Climate controls
The climates of the East African region are
controlled by the:
 Inter-Tropical Convergence Zone (ITCZ) (Kiangi
et al, 1981; Ininda, 1995; Okoola, 1996)
 The Sub-Tropical Anticyclones (Anyamba,
1984).
 the Tropical Monsoons (Findlater, 1969; 1971;
Okoola, 1996)
 Low-Level Cross-Equatorial Jet (Somali Current)
(Findlater 1969, 1971; Kinuthia, 1991)
 and Mesoscale systems (Ogallo, 1988)
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Schematic of general patterns of winds,
pressure and convergence over the East
African region
DJF Season: The
dominant mean wind
flow is North easterly
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JJA Season: The
dominant mean wind
flow is South easterly
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DATA (AOD & GRIDDED WIND)
The monthly long term means (LTM) of AOD
data at 550nm in 1º Χ 1º grid resolution from
the MODIS Terra satellite were used
Web-based trajectory model software from
the NOAA known as HYSPLIT_4 was used to
compute backward and forward air parcel
trajectories (Draxler, R.R. and Rolph, G.D.,
2003)
Backward and forward air trajectories were
calculated for 2 days (48 hours) in order to limit
errors due to uncertainties, Harris et al (2005)
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Methodologies
The methodologies used to achieve the
objectives of the study can be classified
into three categories namely:
 graphical (Hovmöller analysis)
statistical (e.g. Single mass Curves, Time
series, Weighted mean and Std dev.
among others)
and numerical methods (trajectory
Analysis)
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Global & Regional spatial patterns of
MODIS Terra AOD at 550nm
Large land masses contribute
most of dust and carbon related
aerosols to the atmosphere
compared to water masses
Therefore, the northern
hemisphere has more regions
with high AOD values compared
to the southern hemisphere
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Mean monthly AOD amounts around the world
based on observations from the MODIS on
NASA’s Terra satellite
February 2008
July 2008
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Analysis
The analysis
was done from
February 2000 to
December 2008
It can be seen
that there is a
bimodal pattern
in the distribution
of AOD
throughout the
year
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Monthly means for MODIS
Terra AOD at 550nm
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Conference 2014
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The higher peak occurring in July and August
is as a result of the fact that the south easterly
winds that dominate the region of study are well
organized
The region is, also, generally under an
inversion condition for a long period, that is,
June, July and August
This inversion condition does not support
upward movement of air resulting to high
aerosol concentrations at the lower levels of the
atmosphere
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During the rainy months of March, April and
May (MAM) and October, November and
December (OND), it can be clearly noted that the
AOD values are extremely low
 This is due to cloud formation processes and
wet deposition
Wet deposition is the removal of air pollution
components by the action of rain
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Topography is one of the parameters to be
considered in the air quality studies
Surrounding
topography
restricts
the
transport of pollution away from an urban or a
rural area
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When a weak background synoptic circulation
combines with a valley, air temperature
inversion and surrounding orographic blockade,
the result is often a lack of dispersion of
significant lower atmospheric pollutants away
from the area (Berman et al., 1995; Lu and
Turco, 1995; Ellis et al., 2000) as it is the case
in this study
The mesoscale wind systems over those
areas contribute towards enhanced AOD values
found there
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Three case studies of areas seen to have high
AOD values were conducted to determine aerosol
sources and sinks over the East African region
These regions were 2°S, 40ºE; 4°N, 36ºE and
2ºS, 30ºE
Two-day (48-hours) backward and forward air
trajectories starting at height of 500m (red),
1500m (blue) and 3000m (green) above the
ground level (AGL) were computed for 6-hour
intervals in each of the three case studies
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HYSPLIT_4 (version 4) model was used in
the analysis
In this model, it is assumed that the
aerosols follow the air trajectory along the
direction of the wind
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Backward and Forward air trajectory plot for location
2ºS, 40ºE for 48 hours for 20th February and July 2008
respectively
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Backward and Forward air trajectory plot for location
4ºN,36ºE for 48 hours for 20th February and July 2008
respectively
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Conference 2014
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Backward and Forward air trajectory plot for location
2ºS,30ºE for 48 hours for 20th February and July 2008
respectively
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CONCLUSION
The findings of this study suggested that long
distance transport and dispersion of aerosols
through low level winds, 1.4 km and below, is
responsible for the aerosols affecting the East
African region
The transport and dispersion patterns of
aerosols greatly depend on the season of the
year together with the prevailing atmospheric
conditions
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Near Future Plans
A PhD in Climate Change with a bias either
in:
Air pollution
Remote Sensing
Water OR
Agriculture
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SHUKRANI
(Thank you)
T H E W O R L D W E AT H E R O P E N S C I E N C E
CONFERENCE 2014
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